Mercedes 126 Repair – The Perfect Oil Change

The Mercedes 126 is capable of astronomical mileages if properly maintained. To maximize the service life of your vehicle and postpone expensive repairs, frequent oil and filter changes with the appropriate lubricant are essential. While similar to the procedure used for most cars, there are a couple of finer points that 126 owners need to remember. Let’s look at the procedure for the V-8 engines:

No more than 3,000 miles after the last change (and ideally before), take the car for a nice run to get the oil good and hot. Hot oil will flow rapidly from the drain plug, carrying sediments in suspension. Drive the car up on ramps.

To ensure good flow from the drain plug and prevent the formation of any vacuum, remove the oil filter first. Place your drain pan under the filter housing to catch any drips. (There won’t be many. This is a much more elegant design than the hard-to-reach spin-on filters on most modern cars.) Removing the concertina hose to the air intake facilitates access. I also like to loosen the oil filler cap and remove the dipstick.

With your drain pan ready, remove the oil pan plug with a 13mm socket. Cover your hand with a shop rag to protect yourself from the hot oil as the plug is removed from the last couple of threads. Let the oil drain for as long as you can. Overnight is ideal, but 2-3 hours will usually suffice. No, this is not Jiffy-Lube!

While the oil is dripping, replace the oil filter. Filtration is crucial and changing the filter is part of the justification for frequent oil changes. Even if you are using top-quality synthetic oils that could theoretically last a lot longer than 3,000 miles, you do not want to leave the same filter in there too long. The best filters are made by Hengst, with Mann and Mahle close behind. Use the new o-ring for the filter lid and the new steel washer for the long retaining bolt, tightening securely with a 3/8 drive.

When the pan has stopped dripping, replace the oil drain plug, using the new copper crush ring supplied in the filter kit. (There is usually a larger copper ring in these kits that you can ignore as it’s for the 107 SL cars.) Make sure the plug’s threads are clean; replace the plug if it has any burrs and does not turn freely.

Oil capacity is 8.5 quarts. At this stage, add 8.0 quarts. The owners manual contains a viscosity guide for varying climates. Choosing the right brand of oil has become more difficult in the last couple of years; I discuss those issues in another article.

With 8 quarts of oil in the sump, and the filler cap at least partially tight, we’re almost ready to start the engine and drive back off the ramps. But there is a vital step we must take first. To protect the long timing chain, we need to build some oil pressure in the chain tensioner before we fire up the engine. Disable the ignition by disconnecting the green (TD) cable from the ignition control unit. (You could simply remove the coil wire from the spark distributor, but that will send sparks to ground. This method prevents spark formation.) Then crank the engine for 20-30 seconds to build oil pressure. With this done, reconnect the TD cable and start the engine.

With the car off the ramps and level, add the remaining half quart. It will take a while for all the oil to drain into the sump. Whatever reading you then have on your dipstick represents full capacity. Dipsticks can be inaccurate; you cannot always trust the min. and max. markings. Whatever you do, do not overfill. Too much oil in the crankcase will stress the crank seals. When the rear main seal fails you will have drips; when the front crank seal fails you will have more than a drip; neither one is particularly easy to repair.

Check for leaks from the areas you have serviced, and continue to monitor your oil level every week or so. Opinions vary, but I believe in checking the level cold after the car has been sitting overnight. I measure the old oil collected in the drain pan to see exactly how much the engine is using. Older cars tend to have a variety of external oil leaks, but will also have internal leaks past valve stem seals and valve guides. Monitoring consumption will help you decide when those more major repairs can no longer be avoided.